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Using a switching power supply without LDO with 24-bit ADc

Kevin Christie
Prodigy 240 points
Other Parts Discussed in Thread: THS4521, TPS61025, TPS61030

I originally wrote an email to the author of the following article, but it got kicked abck to me, so when you pelase this post just pretend you are him :)

http://www.edn.com/article/520582-Use_a_switching_regulator_to_power_a_high_speed_ADC.php

 

I am working on a new design that will use a 24 bit ADC to measure an output from a MEMS inclinometer that is accurate down to arc seconds.  I am trying to maximize battery life because it is part of a wireless sensor network. 

I came across your article when searching for opinions about using a switching power supply without a typical LDO.  I found it very useful and thank you for writing it. 

I am thinking about using TPS61025, which has amazing efficiency, with one of TIs low data rate 24bit ADCs.  I will be forced to use a level shifter, maybe THS4521 to make the MEMS output of +/-5VDC something the ADC can work with.

This is said in hopes of getting your opinion of my design (which requires a higher accuracy than the one you described in your article) using a switcher without a LDO.

I also wondered when I read the article if the capacitors resonant frequencies were all targeted to be the same or spread out around the switching frequency of the power supply.  Also, did you have to take into account the parasitic effects of the caps based on package size?  I have read that two 1uF caps of different package sizes work better than one 1uF with one 0.1uF of the same package size.

Isn’t there also a way to synchronize the switching of certain power supplies to other devices, such as microcontrollers....and maybe ADCs?

 

 

  • 0
    TI__Guru**** 151080 points

    Some modern DC/DC converters output a clean enough voltage to power sensitive circuitry without excessive filtering or LDOs.  However, you usually don't know how good clean enough is without testing the power supply with your ADC.  So, I will move this post to the data converter forum to see if they have tested this configuration.

    Package size does affect the impedance of the capacitors at higher frequencies.  Most vendors have SPICE programs that can generate these parasitics for their caps.

    Yes, sometimes a power supply's switching is synchronized to a clock.  This helps you know where your noise is--at a specific frequency and occuring at a certain instant in time.  The TPS61030 has such a pin, SYNC.